Methylchloroform is particularly susceptible to decomposition in the presence of metals, especially when aluminum and iron are present. Stabilizers are known which are effective in stabilizing methylchloroform against decomposition induced by contact with iron and aluminum even at elevated temperatures, such as are encountered in vapor degreasing and processes involving purification by distillation.
It is also known, however, that while pure uninhibited methylchloroform at elevated temperatures is relatively inert to the presence of zinc, methylchloroform which is stabilized against decomposition due to contact with other metals, particularly iron and aluminum, tends to decompose badly in the presence of zinc and cause substantial corrosive attack on the zinc metal. Decomposition of the solvent and corrosion of the zinc occur only when the zinc is exposed to the hot vapors of methylchloroform stabilized against iron and aluminum induced decomposition. Zinc below the surface of the boiling solvent will remain virtually unaffected. This is due to the fact that these inhibitors, which stabilize methylchloroform against iron and aluminum, catalyze the attack of methylchloroform on zinc in the boiling vapors of the solvent. This undesirable property causes a restriction of the utility of methylchloroform particularly as a solvent in the vapor degreasing field as galvanized equipment is common and many of the articles to be degreased are zinc or zinc alloy, such as brass or galvanized iron.
An additional need for a stabilizer system to render the hot vapors of methylchloroform inert to zinc surfaces is found in the recovery of used solvent. Methylchloroform may be used in the cold degreasing of metals until it becomes saturated with dirt, grease and other impurities from the metal being cleaned. Spent solvent without the stabilizer system hereinafter proposed cannot be recovered by distillation in the zinc-lined stills commonly used in the industry without damage to the lining thereof.
Methylchloroform where used as a vapor degreasing solvent contains minor amounts of certain additives or stabilizers to prevent decomposition of the solvent induced by its contact with metals such as aluminum and iron. Inhibitors such as 1,4-dioxane alone or in combination with nitromethane, secondary butyl alcohol, or monohydric acetylenic alcohols are commonly employed. Such stabilizers are quite effective in rendering the solvent inert to attack of the metal by the degradation products, but greatly increase the ability of zinc, in the boiling solvent vapors, to cause degradation of the solvent and concomitantly increased attack of the metal. This undesirable property of the solvent may be eliminated, however, by the addition thereto of a vicinal monoepoxide in the amount of from about 0.01 to 5.0 percent by weight of the solvent mixture.
Numerous formulations which contain the epoxide are known to the literature. Thus, for example in U.S. Pat. No. 3,099,694 an epoxide, together with dioxolane and a monoolefin, is employed as stabilizer for methylchloroform. In U.S. Pat. No. 3,265,747 methylchloroform is stabilized with the combination of a lower dialkyl ketone and an epoxide. More recently U.S. Pat. No. 3,974,230 employs methyl butynol, t-amyl alcohol, a nitroalkane and an epoxide in a formulation for stabilizing methylchloroform used in degreasing operations.
It has now been found unexpectedly that the epoxide can be substituted with an alicyclic compound which contains a cyclopropane ring, particularly those tricyclic and quadricyclic compounds having a fused cyclopropane ring in their structure.
A cyclopropane ring compound in which the ring is not fused into a larger cyclic structure is disclosed in our copending application Ser. No. 212,649, filed Dec. 3, 1980. In that disclosure the compound employed is cyclopropyl methyl carbinol.